Add template for 404

web/templates/error/404.html.eex

@@ -0,0 +1,4 @@
+<div class="container">
+  <h1>404</h1>
+  <p>not found</p>
+</div>

web/views/error_view.ex

@@ -1,10 +1,6 @@
 defmodule Asciinema.ErrorView do
   use Asciinema.Web, :view
 
-  def render("404.html", _assigns) do
-    "Page not found"
-  end
-
   def render("500.html", _assigns) do
     "Internal server error"
   end